In order to prove the effects of topographical and geological conditions in behavior of ground motions, a strong motion observation network so called TASSEM has been developed around Toyohashi city, Toyohashi University of
Technology as a center station, that is regarded as marginal area near one of the most vulnerable areas to destructive earthquakes.
Geological and Topographical Aspects [5] [6]
The object area of seismic observation is Toyohashi city located east part of Aichi. Topographical aspects of Toyohashi is generally classified into three areas; (1) the hilly land and the terrace area, (2) the alluvial plain and (3) high lands. The geological feature is made up of the Paleozoic, the Quaternary and the alluvium. The sedimentary layers are consisted of marines without any igneous and metamorphic rocks.
Paleozoic
This area is composed of the Paleozoic Chichibu zone. It is the base of the diluvial formation widely distributed most part of Toyohashi and reveal at the highland of east of Toyohashi. Paleozoic is composed of chart, mudstone, sandstone, etc. and runs generally in direction from east-north-east to west-south-west and has a tendency to incline towards the north or south vertically. Around the object area, it exists about 200m under sea level.
Diluvium
Diluvial formations mainly consist of gravel, sand and silt and form the hilly land and terrace distributed most part of Toyohashi. These are almost horizontally laid on. Caused by the crustal movement called “Atsumi upheaval movement” by Kuroda [7], they incline slightly from south to north.
Alluvium
Alluvium mainly consist of gravel, sand and silt, they have not harden enough yet, and is widely distributed the basin of river.
Location of the Observation System
The arrangement of the observation points is shown in Fig. 3 and 4. Fig. 5 shows the distribution of standard penetration values, N, at each observation point. Table 1 shows site information.
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Accelerometers were installed in December, 1989 ground surface, actually one meter below the surface, of three sites which geological and topographical features are different respectively. POINT 1 is at Hongo Junior High School located on the center of the valley, Umeda River runs east-west and the ground surface is covered with the soft alluvial deposit. Two points locate in the left side of the river, terrace area composed from diluvial layers and called Tempaku-hara Terrace, Tempaku Elementary School as POINT 2, Toyohashi University of Technology as POINT 3. Moreover, as a POINT 4, at Toyohashi Fire Department located in the right side of the river, terrace area composed from diluvial layers and called Takashi-hara Terrace, supplemental observation is being done. From Fig. 5, it is clear that the thickness of soft layer.
Fig. 3 Arrangement of the Observation Points
Fig. 4 Cross Section of the Observation Site
POINT 2 N34°42.5′ E137º24.9′ 21.0m Clayey Sand, Gravel GL-lm
POINT 3, 3B N34°41.9′ E137°24.7′ 39.7m Sand GL-1m, −60m POINT 4 N34°43.4′ E137°24.3′ 25.0m Clayey Sand, Gravel GL
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Observation of base rock motion has been carried out at the layer composed of gravely sand under ground -60m at T.U. T. as POINT 3B since January, 1991 in present system.
Specifications of the Observation System
By employing an advanced electronic technology, seismological recording techniques have made remarkable progress. TASSEM has been ordinarily composed to obtain the fairly distinctive data and easy data management. Table 2 shows the specifications of the system. As a practicable means, the observation center at T.U.T. controls the branch observation points by using of telephone line. Fig. 6 shows outline of TASSEM. TASSEM is provided with the following remarkable functions.
1) By an advanced technology, wide dynamic range and frequency range can be acquired in the system.
2) By watching the state of system operation constantly, the center can get the certainty and reliability of total system operation.
3) Recorded data can be sent to main computer at TUT through the telephone line directly and made visible easily. 4) All seismic observation parameters such as trigger level, record length, sampling frequency, delay time, and correction of time can be easily controlled from the center.
5) As a counterplan against the power failure, the observation can be continued for three hours or more by use of back- up battery.
Table 2 Specifications of the System
Accelerometer Triaxial Force Balance Servo Type Frequency Range 0.02–30 Hz overall
Measurement Range ±1000 gal Dynamic Range 84 dB overall Low Pass Filter 30 Hz, −18 dB/oct.
A-D Converter 14bits, Sampling Rate 100 Hz
Internal Memory IC memory: 1.25 Mbyte, Froppy Disk: 1.25 Mbyte Telemetering Public Telephone Line, Data Transfer Rate 4800bps
Fig. 6 Outline of the system
Observation Results
At present time, several ground motion records have been obtained. Max. acceleration of ground motions observed by TASSEM is shown in Table 3. As one of the largest records, 06:13:07 Sep. 24, 1990, Fig. 7 shows Fourier spectra
calculated and smoothed by using Hamming type window. From the Fig. 7, peculiar peaks are shown in each observation point. The Fourier spectra in POINT 1 and 2 which thickness of soft layer is similar have a flat and wide peak in a short period range. In POINT 3 which thickness of soft layer is comparatively thick, the spectra has the peculiar peak around 3Hz. Up to now, strong motion data have never been obtain at POINT 3B (GL-60m at TUT).
Spectral ratio between the surface and the base, POINT 3B, for microtremor data observed by TASSEM in each
observation point is shown in Fig. 8. In case of POINT 3/POINT 3B, peaks are shown in range from 2 to 3Hz and in case of POINT 1/POINT 3B and 2/3B, large amplification is shown in high frequency range more than 5Hz.
Table 3 Max.Acceleration Observed by TASSEM
(gal) Date Feb. 20, 1990 Apr. 13, 1990 May 17, 1990 Sep. 24, 1990 Sep. 24, 1990
Epicenter N34º46' N35º9' N34°45' N33º6' N33º8'
E139º14' E136º31' E137°37' E138º38' E138º36'
Depth 6km 40km 33km 60km 42km Magnitude 6.5 4.4 3.4 6.6 6.0 Direction EW NS UD EW NS UD EW NS UD EW NS UD EW NS UD POINT 1 – – – 4.1 4.2 1.7 4.3 10.5 2.6 11.2 15.1 3.9 2.3 3.2 1.1 POINT 2 5.5 5.2 2.5 3.7 3.7 2.0 7.1 6.5 4.4 14.9 14.7 6.7 4.1 3.7 1.5 POINT 3 – – – – – – 4.0 2.2 1.7 11.1 15.9 5.4 3.3 4.0 1.3
Fig. 7 Fourier Spectra of the Observation Points
Fig. 8 Spectral Ratio